Method for fixing xerographic images



U i d. S es ,Pam o Ti 2,s94,s40 I 1 v v John H. Dessauer, Pittsford, RobertWi Gundlach, Spencerport, and George R. Mott, -Rochester,IN.Y.,-assignors. to Haloid Xerox Inc.,.a corporation of. New York No Drawing. Application June 14, 1955 Serial No. 515,36

9 Claims. ((11.96-1) This invention relates. to the field of xerography. and, particularly, 'to' an improved method for fixing I a xerographic image on its support.

In the process ofxerography, for example, 'as'disclosed in Carlson Patent 2,297,691,.issued October 6, 1952, a xerographic plate comprising a photoconductive insulating material on a conductive backing is given a uniformv electric charge over its surface and 'is then exposed to. the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the light intensity which reaches them, and thereby creates; an electrostaticlatent image on2'or in the platecoating. Development of the image is effected with a finely divided material such as an electroscopic powder which is brought into contact with the coating and is held thereon electrostatically in-a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerog'raphic powder image. is transferred to a support material to which it is fixedby any. suitable means.

The developing material. employed-may be of any convenient type suitable to'the requirements of .a-particular application. For example, in line copy applications, the developing material may be of-the'type disclosed in Walkup Patent 2,618,551,.issued November 18, 1952, wherein a pigmented powdered resin is combined with coated glass bead carriers and is thereby triboelectricallycharged to a polarity opposite to that of the electrostatic latent image to be developed. Subsequently, the combined material is cascaded over the electrostatic. latent image whereby the resin particles are caused to adhere electrostatically to the charged portions of the. image. Various other pigmented resins, usually having a polystyrene .base, are available to suit specific applications. Incontinuous tone applications of xerography, the developing agent. is'usuallya finely powdered carbon or charcoalthat is electrostatically charged and brought into surface contact with 2,894,840 Patented July 14, 1 959 ticular .type of resin employed; In either 'case', the powder particles of the image are sufliciently softened-to coalesce and to flow into the interstices ofthepaper or support and form a firm bond therewith. Upon cooling of'the image or upon the evaporation of the excess solvent, thebonded pigment forms a permanent record-of the copytobe reproduced.

. In .the case of continuous tone work, in which a charcoal powder is used, the xerographic powder image may be fixed to its support in any of a variety of ways. For example, the powder image may be transferredelectr'o statically or by mechanical pressure to an adhesive coated support that is suificiently tackyto retain the powder, or, the powder image may be electrostatically transferred to a plastic-coated support that is then heated 'sufiic'ien'tly to cause the plastic coating to soften and to adhere tothe powder and firmly bind it to the support on cooling, or,

the powderimage may be transferred electrostatically to a; conventional paper support and then sprayed with Krylon or any other suitable lacquer which, upon drying, is efiective permanently to secure the powder particlesto thesupport. 1 I

Each of the foregoing methods of fixing xerographic powder imageshas been found to be-highly suitable for many types of applications and. each is in wide corn-v mercial use. However, under certain conditions, it" 'is' essential to retain dimensional stability of the support on which the xerographic powder image is to be formed and, also, to avoid coating such supporting materialwitli any substance that may affect the normal utility'of. the supportitself. I r

A typical example of such a combination of conditions arises in the field of'r'ecord controlled machineswherein' coded perforations representing alphabetic and/or'numeric characters are formed on record cards -or.in tape and serve to control various operations of a wide. variety of machines, such as accounting machines, tabulatingma chines, computers, etc. In this field of endeavor it'has been' found highly desirable to form visible xerographic images on the record cards to supplement or duplicate the perforated data indications and thereby amplify or facilitate the use of such records in their regular applications;

However, it has also been found that the conventional xerographic powder image fixing techniques mentioned above cannot conveniently be employed under all. co'ndi:

, tions without occasionally affecting the dimensional sta the electrostatic. latent image on the xerographicplate. 1

veloped-Ito form a xerographic powder particle image,

as "indicated above, and such xerographic powder image is then transferred to another .support material, it is gener'ally required that the xerographicpowder image, subse- .quently be fixed to the paper or other support on which it 'is'formed. The tune of fixing technique that is employed-depends on'therequirements ofiany given installation. For example, the pigmented resins used for line copy-work may be fixed either by heat fusing or by exi fihe t a e tmss here a alie t t he P r.-

bility of a record card. For example, the application of excessive heat or solvent vapor to fix the powder image can, under certain conditions, result in difierential expansion or contraction of portions of'the'c'ard to an extent to make it unfit for use in its normally intended operation. Obviously, any change in the physical characteris: tics of the record cards that wouldaffect theiroperability during card sensing, card feeding, or'card sorting operations cannot be tolerated. I

i The principal object of the present invention is-to provide an improved method for fixing a xerographic powder image on asuppoi't without afiecting the dimensionalgsta: bility or utility of such support. A further object is.- to provide an improved method for fixing a xerographic powder image on a support without the requirement for specialized support material. A further object. of the invention is to provide an improved. method for fixing xer'ographic 'powder images on perforated recordcards. A further object of the invention is to improve the fixing of xerographic powderimages in a rapid, economical, and-eificient manner. v 1 ;-These and other objects of the invention are attained, in conjunction with conventional xero'graphic jpowder images formed on perforated recordcards, by a utilization of, the, difierence .in potential between .the r electrostatic .charge remaining on the xerographic powder images and the charge remaining on the card on which it is formed shortly after the transfer step, by spraying oppositely charged. particles of image-fixing materials in the immediate vinie ity of'the xe'rog'raphic powder image whereby such'fixing material particles are electrostatically attracted selectively to the xerographic powder image ratherithanto the surface of the card. In the case of line coply applications, the image fixing materials. employed may be either of the liquid solvent or low melting point solid solvent type, asv desired. In the former case, the charged liquid solvent particles unite with .and soften the developing material resin immediately upon contact therewith :md cause the resin to bond firmly to the card. In the latter case, the solid solvent particles are physically coupled .to the powder image particles by electrostatic attraction until they are subsequently heated sufiiciently to liquefy the solid solvent particles, whereupon the solvent unites with the developing material resin, as above, to cause the image to be bonded to the card. In the case of continuous tone applications, the image fixing material may bev any convenient type of lacquer that functions to coatthe image particles and simultaneously penetrates the surface of the card, whereby, when the lacquer solvent volatilizes, the image particles remain firmly bonded to thecard. In .each of these cases, the xerographic powder image is fixed in a manner to form a permanent image on the -record card without affecting the dimensional stahility of the card, and a negligible minimum, if any, of the-fixing material is deposited on the card surface so that'it's normal utilization characteristics remain unimpaired.

In the conventional applications of the process of xerography now in general use, the xerographic plate on which the xerographic powder image is to be first formed is uniformly charged with a positive potential. Upon exposure to the subject to be reproduced, this positive potential leaks oif the surface of the photoconductive layer in theexposed areas of the plate but remains on the image areas of the plate and thereby forms an electrostatic latent image of the subject. During development, the-developing material is charged negatively, usually by triboelectriccharging, and is brought into surface contact with the plate containing the latent electrostatic image. The negatively charged developing material is electrostaticallyattracted toand bonded to the positively charged image area of the plate but does not form any electrostatic bond with the exposed area of the plate, so that, after development, a powder particle or xerographic powder image of the subject is electrostatically bonded to the plate.

When'the xerographic powder image is transferred to 'a support material, whether by electrostatic, adhesive, or pressure techniques, as mentioned above, the developing material comprising the transferred powder image retains "substantially all of its negative electrostatic charge. In the case'of a pressure or adhesive transfer, the reason for this is quite obvious since there is no step in either process that has any direct 'effecton the electrostatic charge on the image-forming material. In the case of an electrostatic transfer, the support material is placed over the powderimage on the xerographic plate and :is charged to a high positive potential to attract the negatively charged developing material thereto. However, because of the high resistivity of the developing material, as .explained below, and the limited surface contact of :the dei' elopingmaterial with the supportmaterial, this step neutralizes only a very small percentage of the original charge on the developing material.

After transfer, in the event the xerographic powder image is not fixed immediately, it is found to remain electrostatically bonded to its' support for indefinite periods. Whereas, it has been found that the electrostatic charge, if an placed on the support material bleeds'ofi and is dissipated in relatively short periods, the negative electrostatic charge placed on the developing material remains thereon for much longer periods because of the extremely high resistivity of the developing material as compared to the resistivity of the support material. Although specific resistivities of matetialssuch as xerographic develop ing materials and the usual support materials used therewith are extremely difiicult to measure with any degree of accuracy, empirical data has indicated that most resintype developing materials in general use, which contain from five to ten percent of carbon particles by weight and the remainder in the form of an insulating resin, have resistivities in the range between 10 and 10 ohm cms. On the other hand most support materials appear to have resistivities of from three to six orders of magnitude below the lowest resistivities of the developing materials. Specifically, it can be shown that after transfer of a xerographic powder image or after recharging of a-transferred powder image, the potential remaining on the average support material decays to a point that is immeasurable by conventional electrometer probe methods in a period from ten totwenty seconds after transfer, whereas the powder image portion of the surface probed maintains a residual potential on the order of 20 volts during the same period. ln conventional xerographic practices it is well known that a noticeable difference in deposit from a developing material can be observed between areas differing in potential by about 1 ml volts. Therefore, it is apparent that the above-noted potential difference can be utilized to efiect selective attraction of oppositely charged particles of fixing material to powder image portions of xerographic records without affecting the-nonimage portions thereof.

To this end, the method of the present invention comprises the exposure of tile xerographic powder image and its support to an atmosphere of positively charged particles of fixing material, such as a solvent, binder, or plasticizer for the developing material, during the period after image transfer at whichthe residual charge, if any, has disappeared from the support material but remains upon the developing material. "Preferably, this'is accomplished by supporting the support material with its powder image side down in an atmosphere of finely divided particles of such solvent, binder, or plasticizing material, so that the only effective forces involved are the electrostatic forces acting between the negatively charged powder image material and the positively charged fixing material. In this manner the fixing materials are attracted upwardly to the surface of the powder image material and selectively combine therewith to initiate the fixing action desired, but are not attracted to the-uncharged areas of the support material. The support material is held with image side down 'so that the fixing material cannot deposit thereon as the result of gravitational action.

The particular fixing agent used in any instance depends upon the nature of the developing material forming the xerographic powder image. In practice, the fixing material may be blown :or 'sprayed'into a fixing chamber and a positive charge may be placed on such material either by corona discharge means or :by triboelectric charging, again, depending upon the nature of the specific fixing material employed.

The liquid solvents recommended for use in the method of the invention as applied to the fixing of resin type xerographic' powder images are preferably halogenated hydrocarbons such as trichloroethylene and Freon No. 113, manufactured by the Kinetic Chemical Division of E. I. du Pont de "Nemours & Company of Wilmington 98, Delaware. Although various other solvents would serve the same purpose they are in general not as desirable from the standpoint of cost, availability, infiammability, and toxicity. For the fixing -of charcoal'type xerographic powder images encountered in continuous-tone work, an

Inc., of Philadelphia 46, Pa., is considered most desirable, although other liquid lacquers may be employed.

In the event the support material may be subjected to a limited amount of heat without aflecting its dimensional stability, it may be preferred to use any of the various low melting point solids that can function as solventsor plasticizers for the developing material upon the application of heat. In such cases, the particular low melting point solid chosen would be ball milled 'or powdered in an air jet. pulyerizer to the proper degree of'fineness and then given a positive electrostatic charge either by corona discharge or triboelectrically as it is blown into the fixing chamber. These powder particles would be attracted electrostatically to the xerographic powder image and would be electrostatically bonded thereto upon contact therewith. Thereupon, the support with the combined powder image material and fixing material thereon is, subjected to a heating cycle of sufficient intensity merely to liquefy the particular low melting pointsubstance that is used. When this occurs, the solvent unites with the powder image material and bonds it to the card in the same manner as the liquid solvents discussed above. I

For this purpose, any of a wide variety of organic materials may be used which havea plasticizing or solvent efiect upon the particular developing material in question, the only requirement being that the selected material be solid at room temperature and that its melting point preferably be in therange from 150 F. to 225 In general,a variety of chlorinated or aromatic hydrocarbons are considered suitable for this purpose although any organic material having some affinity for the developing material, such as similar solubility characteristics, is probably also useful. Specific compounds that are considered suitable for this purpose are as follows: diphenyl phthalate, triphenyl phosphate, N-ethyl para toluene sulfonamide, N-cyclohexyl para toluene sulfonamide, meta nitrobiphenyl, para nitrobiphenyl, arochlors (chlorinated p0lyphenyl).f

. The method of the invention, as described above, contemplates the use of conventional xerographic powder image transferring techniques and commercially available types of developing materials. However, it is apparent thatvarious modifications may readily be employed in such a method. For example, by transferring the xerographic powder image at higher than conventional potentials, or by recharging a support on which a xerographic powder image had been formed, or by using difierent proportions of carbon black or similar material inthe developing material, itis readily possible to obtain even greater difierences in residual potentials than those mentioned above. It will be apparent that such modifications are within the scope of the present invention as defined in the appended claims.

Although the method of the invention as disclosed is applied to the fixing of xerographic powder images on perforated record cards, it will be apparent to those skilled in this or related arts' that the techniques of the present invention are equally applicable to any situation in which the dimensional stability or other utilization characteristic of the xerographic image support material is of critical importance.

What is claimed is:

1. The method of fixing a xerographic powder image on its support, wherein the powder particles comprising the xerographic powder image have a residual electrostatic charge, said method comprising the steps of producing an atmosphere of electrostatically charged finely divided particles of image fixing material which is a solvent for the xerographic powder image particles, the electrostatic charges on said particles being of opposite polarity to the residual electrostatic charge on said powder image, and exposing said xerographic powder image to said atmosphere, whereby said image fixing material particles are electrostatically attracted to the xerographic powder image particles and are combined therewith to eifect image fixing solely by electrostatic attraction of the fixing particles to the image particles.

2. The method offixing a xerographic powder image on a support, wherein the powder particles comprising the xerographic powder image have a residual electrostatic charge, said method comprising the steps of producing an atmosphere of electrostatically charged finely divided particles of image fixing material which is a solvent for the xerographic powder image particles, the electrostatic charges on said particles being of opposite polarity to the residual electrostatic charge on'said powder image, and exposing said xerographic powder image to said atmosphereafter the residual electrostatic charge on said support has decayed, whereby said image fixing material particles are electrostatically attracted to the xerographic powder image particles and are combined therewith to efiect image fixing solely by electrostatic attraction of the fixing particles to the image particles.

3. The method of fixing a xerographic powder image on its support, wherein the developing material forming the xerographic powderimage has a higher resistivity than the material of said support, wherein the powder'particles comprising the xerographic powder image and the support have a residual electrostatic charge, said method comprising the steps of producing an atmosphere of electrostatically charged finely divided particles of image fixing material which is a solvent-for the xerographic powder image particles, the electrostatic charges on said particles being of opposite polarity to the residual electrostatic charge on said powder image, and exposing said xerographic powder image to said atmosphere after the residual charge on said support material has decayed, whereby said image fixing material particles are electrostatically attracted to the xerographic powder imageparticles and are combined therewith to effect image fixing solely by electrostatic attraction of the fixing particles to the image particles.

4. In the process of xerography wherein an electrostatically-charged xerographic plate is exposed to a projected image to form a latent electrostatic image on the plate corresponding to the projected image and the plate is developed with oppositely charged particles of developing material that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and having a residual electrostatic charge of the same polarity as the developing material and wherein the xerographic powder image is transferred to a support material, the improvement comprising the steps of producing an atmosphere of electrostatically-charged finely-divided particles of image fixing material which is a solvent for the xerographic powder image particles, the electrostatic charges on said particles being of opposite polarity to the residual charge on said xerographic powder image, and exposing said xerographic powder image to said atmosphere, whereby said image fixing material particles are selectively attracted to said xerographic powder image and are combined therewith to effect image fixing solely by electrostatic attraetion of the fixing particles to the image particles.

5. In the process of xerography wherein an electrostatically-charged xerographic plate is exposed to a projected image to form a latent electrostatic image on the plate corresponding to the projected image and the plate is developed with oppositely charged particles of developing material that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and having a residual electrostatic charge of the same polarity as the developing material and wherein the xerographic powder image is electrostatically transferred to a support material by applying an electrostatic charge opposite to the residual charge to the support material while it is in contact with the xerographic powder image and separating the support material from the xerographic plate, the improvement comprising the steps of producingan atmosphere of electrostatically-charged finely-divided particles of image fixing material which is a solvent for the xerographic powder image particles, the electrostatic charges on said particles being of opposite polarity to the residual charge on said xerographic powder image, and exposing said xerographic powder image to said atmosphere prior to the decay of the residual charge on the xerographic powder image, whereby said image fixing material particles are selectively attracted to said xerographic powder image and are combined therewith to effect image fixing solely by electrostatic attraction, of the fixing particles to the image particles. 7

6. In the process of xerography wherein an electrostatically-charged xerographic plate is exposed to a projected image to form a latent electrostatic image on the plate corresponding to the projected image and the plate is developed with oppositely-charged resin-base particles that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and having a residual electrostatic charge of the same. polarity as the developing material and wherein the xerographic powder image is transferred to a support material, the improvement comprising the steps of producing an atmosphere of electrostatically-charged finelydivided particles of resin solvent material which is a solvent for the xerographic powder image particles, the electrostatic charges on said particles being of opposite polarity to the residual electrostatic charge on said powder image, and exposing said xerographic powder image to said atmosphere, whereby said resin solvent particles are selectively .attracted to the resin-base particles of said xerographic powder image and are combined therewith to effect image fixing solely by electrostatic attraction of the fixing particles to the image particles.

7..In the process of Xerography wherein an electrostatically-charged xerographic plate is exposed to a projected image to form a latent electrostatic image on the plate corresponding to the projected image and the plate is developed with oppositely-charged powdered charcoal particles that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and having a residual electrostatic charge of the same polarity as the developing material and wherein the xerographic powder image is transferred to a support material, the improvement comprising the steps of forming an atmosphere of electrostatically-charged finelydivided particles of charcoal binder material, the electrostatic charges on said particles being of opposite polarity to the residual electrostatic charge on said powder image, and exposing said xerographic powder image to said atmosphere, whereby said charcoal binder particles are selectively attracted to the powder charcoal particles of said xerographic powder image to effect binding of the charcoal particles to the support material solely by electrostatic attraction of the binder particles to the charcoal particles.

8. In the process of xerography wherein an electrostatically-charged xerographic plate is exposed to a projected image to form a latent electrostatic image on the plate corresponding to-the projected image and the 'plate is developed with oppositely-charged resinous particles that areattracted to the latent image to for-In a xerographic powder image corresponding to the projected image and havinga residual electrostatic charge of the same polarity as the developing material and wherein the xerographic powder image is transferred to a support material, the improvement comprising the steps of producing an atmosphere of electrost'atically-charged finelydivided particles of powdered resin solvent material which is a solvent for the xerographic powder image particles and has a melting point'in the range from F. to 225 F., the electrostatic charges on said particles being of opposite polarityto the residual electrostatic charge on said powder image, and exposing said xerographic powder image to said atmosphere, whereby said powdered resin solvent particles are selectively attracted to the resinous particles of said xerographic powder image solely by electrostatic attraction, and applying heat to liquefy the powdered resin solvent.

9. In the process of xerography wherein an electrostatically-charged xerographic plate is exposed to a projected image to form a latent electrostatic image on the plate corresponding to the projected image and the plate is developed with oppositely charged particles of developing material that are attracted to the latent image to form a xerographic powder image corresponding to the projected image and wherein the xerographic powder image is transferred to a support material having a lower resistivity than the developing material, the improvement comprising the steps of imposing a uniform electrostatic charge on said xerographic powder image and said support, producing an atmosphere of electrostaticallycharged finely-divided particles of image fixing material which is a solvent for the xerographic powder image particles, the electrostatic charges on said particles being of opposite polarity to the electrostatic charge on said powder image and support, and exposing said xerographic powder image to said atmosphere after the electrostatic charge on said support material has decayed, wherebysaid image fixing material particles are selectively attracted to said xerographic powder image and are combined therewith to effect image fixing solely by electrostatic attraction of the fixing particles to the image particles.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF FIXING A XEROGRAPHIC POWDER IMAGE ON ITS SUPPORT, WHEREIN THE POWDER PARTICLES COMPRISING THE XEROGRAPHIC POWDER IMAGE HAVE A RESIDUAL ELECTROSTATIC CHARGE, SAID METHOD COMPRISING THE STEPS OF PRODUCING AN ATMOSPHERE OF ELECTROSTACIALLY CHARGED FINELY DIVIDED PARTICLES OF IMAGE FIXING MATERIAL WHICH IS A SOLVENT FOR THE XEROGRAPHIC POWDER IMAGE PARTICLES, THE ELECTROSTATIC CHARGES ON SAID PARTICLES BEING OF OPPOSITE POLARIITY TO THE RESIDUAL ELECTROSTATIC CHARGE ON SAID POWDER IMAGE, AND EXPOSING SAID XEROGRAPHIC POWDER IMAGE TO SAID ATMOSHPERE, WHEREBY SAID IMAGE FIXING MATERIAL PARTICLES ARE ELECTROSTATICALLY ATTRACTED TO THE XEROGRAPHIC POWDER IMAGE PARTICLES AND ARE COMBINED THEREWITH TO EFFECT IMAGE FIXING SOLELY BY ELECTROSTATIC ATTRACTION OF THE FIXING PARTICLES TO THE IMAGE PARTICLES. 